Pitting and Crevice

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  • Corrosion Forms II

    Pitting and Crevice Corrosion

    Dr. Zuhair M. GasemME, KFUPM

  • Dr. Z. Gasem ME472-062

    KFUPM2Forms of Electrochemical Corrosion

    Four categories of corrosion forms:

    General corrosion (uniform corrosion over the whole anode) Localized Corrosion (corrosion at isolated areas in the anode)

    Galvanic corrosion pitting corrosion Crevice corrosion

    Metallurgically Influenced Corrosion (corrosion is related to microstructure) intergranular corrosion of stainless steel

    Mechanically Assisted Corrosion (corrosion is accelerated due to mechanical factors) Erosion corrosion Corrosion Fatigue

    Environmentally Assisted Cracking (corrosion results in cracking) Stress corrosion cracking Hydrogen damage

  • Dr. Z. Gasem ME472-062

    KFUPM3Forms of Corrosion

  • Dr. Z. Gasem ME472-062

    KFUPM4Pitting Corrosion

    Metals and alloys can be active (no protective passive film) or passive (passive film 10-30 nm in thickness)

    Pitting corrosion : rapid corrosion penetration at small discrete areas where passive film breaks down and appears as small diameter holes (0.1-5 mm). The remaining surface is not attacked.

    Pitting corrosion is common in passive metals above pitting potential (Ep)

    Weight loss is negligible

    Pits

    passivefilm

  • Dr. Z. Gasem ME472-062

    KFUPM5Pitting Corrosion

    Examples: Pitting of stainless steel, Ni

    alloys, Al alloys in aerated seawater or acids containing Cl-ions

    Steel buried in the soil corrodes with the formation of pits.

    Pitting of steels and aluminum in alkaline solutions containing Cl-

    Pitting can occur in any alloy that form a protective film and exposed to stagnant solution containing Cl-

    Example: 316 SS centrifuge head exposed to CaCl2 solution showing deep pits

  • Dr. Z. Gasem ME472-062

    KFUPM6Pitting Corrosion

    Right: pitting in carbon steel pipe carrying strong acid.

    Below: deep pits on a carbon steel in HCl.

  • Dr. Z. Gasem ME472-062

    KFUPM7Pitting Corrosion

    Top: pits in stainless steel valve plate (stagnant solution).

    Bottom: internal pits in steel pipeline carrying water with dissolved CO2

  • Dr. Z. Gasem ME472-062

    KFUPM8Pitting Corrosion

    Conditions favoring pitting: Stagnant solutions

    pitting occurs during shutdown period more than during normal operation During shutdowns, solutions in pipes, pumps, and tubes are stagnant and

    allow pits formation. Acidic solutions Presence of Cl and Br (halides) High temperature rough surface finish Commonly occur in passive metals (such as stainless steels) Presence of surface deposits (deposits form localized concentrated

    environments around them) Rupture of protective coatings Pits form in gravity direction

  • Dr. Z. Gasem ME472-062

    KFUPM9Pitting Corrosion

    Why pitting corrosion is dangerous? Pitting corrosion is not easily detected and

    evaluated and thus more dangerous than general corrosion or galvanic corrosion.

    Pits can grow and become like cracks Can lead to failure by perforation with

    minimum weight loss Pits formation may take place after a long

    period of initiation (tricky) Aluminum parts in the space shuttle Apollo

    were rejected because they contained pits having a diameter of a human hair.

  • Dr. Z. Gasem ME472-062

    KFUPM10Pitting Corrosion

    Difficulty of detection Different misleading

    pit (hole) shapes Small weight loss Cant use thickness

    measurement Small pits are hard to

    observe visually Corrosion products

    may mask pits

  • Dr. Z. Gasem ME472-062

    KFUPM11Stages in Pitting Corrosion

    Two stages in pitting: initiation and propagation Pits initiation at film break downs due to

    Mechanical scratch Localized High Cl- concentration Localized failure of coatings or paints At metal microstructural inhomogenity (inclusions in SS)

    Pits initiation at surface deposits or scratches of paints Most initiated pits become unstable and do not grow if

    the passive film reforms or the highly concentrated environments are washed off

    If passive film reformation is not possible (high concentration of Cl-), then a pit will grow at faster rate due to autocatalytic effect.

  • Dr. Z. Gasem ME472-062

    KFUPM12Autocatalytic Nature of Pitting

    In the propagation stage of pits in SS in aerated salt water (seawater): Oxygen is depleted inside the pit

    and oxygen reduction takes place around the pit

    Anodic dissolution of the metal produces M+ ions inside the pit

    Cl ions diffuse toward the pit to neutralize cations charge.

    Cl and M+ inside the pit combine and produce more H+ by hydrolysis

    (M++Cl)+H2OMOH+H++Cl The electrolyte becomes more acidic

    inside the pit and more corrosive because higher H+ and Cl

    Reach autocatalytic stage where pitgrowth follows d = ktn.

    k is a constant and n ranges from 0.3 to 0.8

    time

    d (Pit depth)

    initiation Propagation

    O2 + 2H2O + 4e-4 OH-

    Parabolic growth

  • Dr. Z. Gasem ME472-062

    KFUPM13Effect of Temperature

    Pitting is strongly influenced by the service Temperature

    In stainless steels there is a critical pitting temperature below which

    pitting will not initiate for each stainless steel alloy 304SS (critical pitting temperature = -2.5 C) 317SS (critical pitting temperature = -0 C) Pitting resistance is improved by increasing Cr

    (>18%) and Mo (2-4%).

  • Dr. Z. Gasem ME472-062

    KFUPM14Temperature effect

    Three alloys are tested for pitting resistance by immersion for 24 hr in very aggressive solution. All alloys have high

    resistance to pitting at 25 C Alloy G: Ni-20Fe-22Cr-6Mo

    Shows pitting at 70 C Alloy 625: Ni-22Cr-9Mo

    Shows pitting at 102 C Alloy C-276: Ni-16Cr-16Mo-

    4W No pitting until 102 C

  • Dr. Z. Gasem ME472-062

    KFUPM15Pitting Evaluation and Control

    Use of standard charts to estimate pitting density (number of pits per cm2of metal surface)

    Pitting evaluation is given according to standard charts A-4 pit

    density=100,000/m2 B-3 pit surface size = 8

    mm C-2 Pit depth = 0.8 mm

    Use pit opening diameter Average pit depth

  • Dr. Z. Gasem ME472-062

    KFUPM16Control and Prevention of Pitting Corrosion

    Reduce Cl- content in the electrolyte Lower acidity of solution, lower O2 Shot peen the surface. Passivate stainless steels (SS) by washing with 20%

    HNO3 to give it a strong passive film. Replace 304 SS with 316 which contains higher Mo Avoid stagnant solutions in tanks, tubes, and pipes Redesign to ensure proper drainage

  • Dr. Z. Gasem ME472-062

    KFUPM17Concentration Cells

    Variation in the electrolyte in: dissolved ion concentration dissolved O2 pH temperature

    create a concentration cell in the electrolyte where some region of the electrolyte will favor cathodic reactions and others favor anodic reactions.

    Corrosion of storage tanks: corrosion will be localized at the boundary between two electrolytes having different ion or O2 concentrations.

    The electrode potential becomes more +ve as the metal ions concentration increases (see Nernstsequation)

  • Dr. Z. Gasem ME472-062

    KFUPM18Oxygen Concentration Cell

    In oxygen concentration cell, for example, cathodic reactions are favored in region having high concentration of Oxygen.

    Example 1 : a drop of water on a metal surface. The cathodic reaction is:

    O2 + 2H2O + 4e- 4 OH-

    In neutral solution (pH=7) e=0.82+0.059/4*log PO2 e is more +ve PO2

    The drop surface dissolves O2 from the air and will have high concentration of O2.

    At the center of the drop, the O2 concentration is low.

    The metal at the periphery of the drop (high O2) will act as the cathode (more +ve e).

    The metal at the center of the drop (low O2) will act as the anode.

    This creates an oxygen concentration cell or air differential cell which localizes corrosion.

    Oxygen concentration cell is responsible for many corrosion problems.

    Example 2: oxygen concentration cells on the legs of offshore steel structure

  • Dr. Z. Gasem ME472-062

    KFUPM19Differential aeration cell

    Cathodic reaction:

    O2 + 2H2O + 4e-4 OH- Corrosion is frequently

    caused by concentration difference in O2 from air.

    Oxygen solubility in water at RT is very small (8 ppm) and decreases with temp.

    Cathode: at high conc of O2 near the top surface of tank or sea surface

    Anode at low conc of O2below the surface

  • Dr. Z. Gasem ME472-062

    KFUPM20Differential aeration cell

    corrosion in stainless steel thermometer pocket in sea water cooled steam condenser.

    A stagnant layer of sea water formed along the stem on the downstream side

    Differential aeration cell

  • Dr. Z. Gasem ME472-062

    KFUPM21Crevice Corrosion

    Crevice: narrow opening or small gap between two contacting surfaces.

    Crevice corrosion: localized corrosion resulting from a concentration difference b/w the electrolyte within the crevice and the electrolyte outside the crevice due to stagnation of electrolyte.Possible concentration difference in: O2 H+ (acidity) Cl-

    Low PO2 , high [Cl-] and H+

    High PO2 ,low [Cl-] and H+

    (0.025-0.1 mm)

  • Dr. Z. Gasem ME472-062

    KFUPM22Crevice Corrosion

    Inside the crevice: low concentration of O2, and high concentration of H+, and ions such as Cl-(more